As portable electronic devices, such as pagers, become more advanced, additional digital features are being incorporated within. For example, some modern pagers now include clock functions for keeping the time of day, as well as large volatile digital memory circuits (or RAM) for storing accumulated message information. Since such devices are portable, they operate on a primary battery. In the event of the absence of power from the primary battery, either by the battery going dead or the battery being removed by the user for the purpose of inserting a new battery, devices without backup battery functions loose important digital information such as the time of day and accumulated information stored in volatile memory. Conventionally, in order to protect against this loss of information, a backup battery has been used to provide power for the device when power from the primary battery is absent.
Prior art backup battery systems include systems which use long life lithium batteries. However, such batteries are large and therefore difficult to incorporate within miniature portable electronic devices, such as pagers. Also, such batteries do not store sufficient energy to provide for backup functions over the expected life of the product. Thus, the backup battery would need to be periodically replaced. This results in an inconvenience to the user of the device, as well as the additional expense of providing a means for easy removal and replacement of the backup battery. Therefore, it is desirable to use a rechargeable backup battery which may be both smaller in size, thereby providing for improved miniaturization, and permanently installed within the device, thereby eliminating the inconvenience of replacement and the means for replacement. Known pagers have incorporated rechargeable backup batteries. FIG. 1 represents a known pager incorporating a rechargeable backup battery. Primary battery 10, is a common removable AA or AAA battery having a DC output voltage ranging from 1.7 when new to 1.0 when discharged or dead. Voltage multiplier 20 boosts the battery voltage to a regulated 2.6 volt output 30. The operation of the voltage multiplier 20 is described in detail in U.S. Pat. Nos. 4,606,076 and 4,634,956 both to Davis et al, and assigned to the assignee of the present invention. Control 22 generates an 83 kHz duty cycle modulated signal which drives switching transistor 23 which either grounds or floats inductor 24. When floating, the fly-back energy of inductor 24 is conducted to diode 27 and stored on capacitor 28. Control 22 modulates the duty cycle of the signal in order to maintain a regulated 2.6 volt output. Under normal loads, inductor 24 operates in "continuous conduction", thereby producing an 83 kHz waveform at node 29 having a low voltage of ground and a high voltage a diode drop greater than the 2.6 volt regulated output voltage at node 30. Thus, the waveform at node 29 is substantially a 2.6 volt peak to peak waveform.
Decoder 40 receives operating power from node 30 at input 32. Although decoder 40 is capable of operating at voltages higher than 2.6 volts, operating the decoder at a reduced voltage reduces the power consumed from the primary battery, thereby extending the battery life. Decoder 40 includes a microcomputer for performing paging functions well known to those familiar with the art, such a decoder is described in U.S. Pat. No. 4,835,777 to DeLuca et al, and assigned to the assignee of the present invention. Decoder 40 sends a signal to control means 22 through line 42 indicating whether voltage multiplier 20 is to operate in a high or low power mode. The high power mode indicates that a high power load connected to node 30 may be enabled, the high power load may result from operating the decoder at a substantially higher clock speed for certain decoding functions. The high power mode causes control 22 to provide additional base drive current to transistor 23. Otherwise the low power mode is used resulting in reduced base drive current in transistor 23, thereby conserving power drawn form primary battery 10.
Decoder 40 also receives and processes paging signals having address and subsequent message information modulated upon RF signals received and demodulated by receiver 45. In response to the detection of a predetermined address, decoder 40 then stores subsequent message information in volatile memory (not shown) within decoder 40 and displayed on display 48. Decoder 40 also comprises a means for keeping time of day and date information (not shown).
Backup battery 50 is fixed within the pager and supplies operating power to decoder 40 when power from the primary battery is no longer available. Backup battery 50 is preferably a rechargeable lithium battery such as part number SL621, manufactured by Seiko Instruments, Inc. Power to recharge battery 50 passes through resistor 52 which limits the current drawn from node 30. The recharge voltage of battery 50 approaches the regulated voltage at node 30 as the voltage across resistor 52 approaches zero. Thus, the maximum charged voltage of the backup battery is the voltage of node 30 when the voltage across resistor 52 equals zero.
Battery absent detector 60 detects the unavailability of power from the primary battery and switches transistor 62 on, thereby providing power from the backup battery 50 to node 30. Battery absent detector also sends an absent signal 64 to decoder 40 indicating the absence of power from the primary battery, thereby causing the decoder to operate in a low power mode while maintaining message information stored in the volatile memory and time keeping functions. When power is again available from the primary battery, detector 60 sends a present signal 64 to decoder 40 and normal paging operations are resumed. Absence detector 60 also senses the voltage of backup battery 50 and supplies a reset signal 64 to decoder 40 indicating that the voltage is too low to guarantee proper decoder operation. This voltage is typically 2.2 volts. In response, decoder 40 erases messages in volatile memory and resets the time keeping functions.
Thus, during backup operation, the prior art backup battery system provided for the battery voltage to range from 2.6 to 2.2 volts before backup operations are terminated by the decoder and important information is reset. Since this range corresponds to the time the pager operates in the backup mode, it is desirable to increase this range in order to increase the time a pager can operate in the backup mode. Also, it is desirable to increase the electrical functionality while the device is operating from the backup battery. Such increased functionality includes the display of time information or other status information on display 48 when the primary battery is removed or dead. This increased functionality draws extra power from the backup battery, thus it is desirable to provide a backup battery system having improved backup battery capacity.
Also, at times users replace a dead primary battery with a weak primary battery thereby allowing insufficient time for recharging the backup battery before the weak primary battery is itself dead. Thus, what is needed is a backup battery charging system capable of rapidly recharging a backup battery from a primary battery. Also, the backup system should not interfere with the measurement of operating parameters of the device, such as measurement of current drain or radio characteristics.